Electric power is generated efficiently in gasification combined cycle (GCC) systems in which coal or other carbonaceous material is gasified using oxygen to provide synthesis gas containing the combustible components hydrogen and carbon monoxide. The synthesis gas, which also contains carbon dioxide and in some cases methane, is fired as fuel to a gas turbine system which drives a generator to produce electric power. Hot turbine exhaust is passed to a heat recovery system to produce high pressure steam which is expanded through a steam turbine to drive another electric generator to produce additional power. Such gasification combined cycle systems generate electricity in an efficient and environmentally sound manner.
The production of chemicals or liquid fuels from a portion of the synthesis gas in a gasification combined cycle system is well known and has the advantages of common operating facilities and economy of scale in the coproduction of electric power and chemicals. Several references in the background art describe existing technology for combined chemical plant/GCC power plant operations.
U.S. Pat. No. 5,179,129 describes the integration of a multi-stage liquid phase methanol plant with a standard GCC system. Excess heat of reaction from the methanol reactor is used to heat compressed synthesis gas reactor feed and boiler feed water, or to generate steam for the generation of additional electric power. U.S. Pat. No. 4,946,477 describes a liquid phase methanol/GCC system without specific heat integration between the methanol and GCC plants.
U.S. Pat. No. 4,676,063 describes a methanol synthesis/GCC system with multiple parallel modules for operating flexibility. Heat from the acid gas removal system of the GCC plant is sent to the methanol plant to saturate the syngas feed stream with water before employing a water-gas shift to increase hydrogen in the feed stream to the methanol reactor. No heat from the methanol plant is used in the GCC system. U.S. Pat. Nos. 4,663,931 and 4,665,688 describe essentially the same system in which a portion of the methanol product provides feed for the production of acetic acid or vinyl acetate respectively. In both cases heat of reaction from the methanol plant is used to generate steam.
U.S. Pat. No. 4,631,915 describes essentially the same system as U.S. Pat. No. 4,676,063 with the addition of a molten iron bath gasifier integrated with a primary hydrogenating coal gasifier to produce syngas for a methanol plant. No specific methanol plant to GCC heat integration is specified in this patent. U.S. Pat. No. 4,608,818 describes essentially the same system as U.S. Pat. No. 4,676,063 without acid gas removal system heat integration to saturate the methanol plant feed stream; no specific methanol plant/GCC heat integration is specified. U.S. Pat. No. 4,590,760 describes essentially the same system as U.S. Pat. No. 4,676,063 with added cooler-saturator loops to cool the raw syngas stream from the main gasifier before entering the acid gas removal system and to saturate the syngas stream entering the methanol plant. There is no integration in which heat from the methanol plant is utilized in the main GCC system.
U.S. Pat. No. 4,277,416 describes a basic methanol plant with syngas feed from a coal gasifier or steam methane reformer with no specific heat integration. This patent also describes an operation in which some of the syngas is combined with effluent nitrogen from an air separation plant to provide feed to a urea plant.
U.S. Pat. No. 4,273,743 describes a chemical reaction system (preferably ammonia synthesis) providing heat to a semi-closed Brayton cycle power plant for the integrated production of power and chemical product. This patent specifically teaches methanol synthesis as well, but does not teach application to open Brayton cycle power generation of the type used in GCC plants.
UK Patent Application GB 2,075,124 describes an integrated GCC/methanol plant with methanol co-firing to enable fast startup and superior power load following capabilities. The production of steam in the methanol plant and use of this steam for power generation in a steam turbine is disclosed.
South African Patent Application 853,010 describes a system for the production of ammonia or methanol integrated with a coal or heavy oil gasification combined cycle power plant. In this system, the cooling of the syngas stream before sulfur removal is used to generate steam for increased power generation in the steam turbine section of the combined cycle plant. There is no heat integration between the ammonia or methanol plants and the power plant.
The capacity of the main air compressor of a gas turbine power plant can be increased by cooling the air feed to increase gas density and/or additionally humidifying the air to increase mass flow rate. This is usually accomplished by a separate compressor-driven heat pump refrigeration system which adds significant capital cost and draws substantial electric power from the overall plant. U.S. Pat. No. 4,424,667 describes the use of a heat pump powered by the gas turbine output to pre-cool the inlet air to the main gas turbine air compressor. U.S. Pat. Nos. 3,877,218 and 3,788,066 describe the use of a separate compressor driven refrigeration system to pre-cool the air fed to the main gas turbine compressor. U.S. Pat. No. 3,796,045 describes the use of a separate compressor drive refrigeration system and a supercharging inlet fan to pre-cool and pressurize the air to the main gas turbine compressor. These methods for precooling gas turbine air compressor feed typically require expensive, complicated equipment with significant power draw from the gas turbine system.
In the background art described above, heat from the reaction section of a combined GCC/chemical production system is utilized in the GCC system to generate steam for use in the steam turbine. Other uses of the reaction heat in the GCC system are not disclosed. Similarly, direct refrigeration of GCC streams is disclosed in the background art, but use of recovered refrigeration from a chemical production system integrated with a GCC system is not taught.
GCC systems have environmental advantages over traditional power plants which utilize liquid or solid carbonaceous fuels, and oxygen-derived synthesis gas is an attractive feedstock for the coproduction of chemical or liquid fuel products and electric power. New integrated GCC/chemical coproduction plants will be installed and operated in coming years because of favorable environmental and economic advantages, and methods to improve the efficiency and degree of integration of such plants are desirable. The invention disclosed in the following specification and defined in the appended claims offers methods for such improvements to GCC/chemical coproduction plants.